EP-A-0.557.935 discloses a process of this type, in which the first stream is of the air to be distilled, the second stream is of the nitrogen at medium pressure supercharged by the supercharger 110 and the third system is a residual gas expanded in the turbine 113 which is coupled with the supercharger.
DE-A-28.54.508 discloses a process of this type, in which a portion of the air is first supercharged in the supercharger then expanded in a turbine coupled with the supercharger, then sent to the low pressure column of a double column.
These processes, increasing the expansion load in the turbine, permit reducing the work expanded flow with a given production of cold or increasing the production of cold with a constant expanded flow rate with, in general, an improvement of the performance of the installation.
In certain cases, there is no simple means for carrying out a process of this type or simply of recovering with a supercharger the mechanical energy produced by the turbine. For example, when the turbine expands nitrogen withdrawn from the medium pressure column of an air separation installation, it is less interesting to reheat this nitrogen, available at low temperature so as to compress it and then cool it to send it to the turbine, which increases the capital cost of the heat exchange line.
DE-A-25.57.453 discloses a solution to this problem with a process according to which the second stream constitutes a fraction of the first stream, which is the air to be distilled. The supercharger 5 of the second stream is coupled to a turbine 6 which serves to expand the gaseous nitrogen at medium pressure (the third stream).
Again in the case of DE-A-28.54.508, it can happen that the desired cold production requires a work expanded flow less than or equal to that which permits distillation. In this case, the reduction of the work expanded air flow gives rise to no saving and there is accordingly no interest in increasing the intake pressure of the turbine because there is no interest in reducing the work expanded flow rate that results.
The prior art thus teaches recovering the energy of the turbine with a generator or to dissipate the energy with an oil brake.